Such measuring arrays, e.g., in the form of a sensor, are used to determine the concentration of a gas, e.g., oxygen, in a mixture with other gases based on the radiation absorption, which is characteristic of the particular substance and is concentration-dependent. Such an optical measuring array contains as its essential elements one or more laser sources, optical elements for guiding the beam, as well as a measured gas sample holder and one or more radiation detectors.
Interferences, which markedly limit the sensitivity of such an optical measuring array, are generated by multiple reflections of the laser beam on surfaces positioned between laser sources and radiation detectors. It has been known that this adverse effect can be markedly reduced by obliquely positioning the optical elements in the beam path. However, this measure is insufficient for improving the signal resolution in the case of a high-resolution system with weak absorptions, such as that of oxygen. In addition, a lengthy averaging of the signals, which would also reduce the effect of the interferences, is not possible in the field of medicine, because rapid signal rise times are desirable for measurements that are resolved for individual breaths.
Various suggestions have been made to improve the sensitivity of sensors and the resolution of the measured signal. One prior-art solution is to increase the length of the absorption path. To nevertheless prevent an impairment in the time resolution, the volume of the gas sample holder must be maintained, i.e., the diameter of the sample holder must be markedly reduced. This leads to problems in focusing and guiding the laser beam.
An array for averaging out the interferences has become known from U.S. Pat. No. 4,934,816. The essential component of this prior-art array is an active optical element, which is arranged in the course of the beam path and oscillates along the optical path. The drawback of this prior-art array is the technical effort needed to achieve the tightness of the absorption cell with the oscillating optical element, on the one hand, and additional transmission losses which due to the necessary multiple deflections of the laser beam, on the other hand.